1. Field of the Invention
The present invention relates generally to a method for detecting the presence of foam forming compounds in aqueous solutions. More particularly, the present invention relates to a method which detects the presence of specific foam forming compounds in an aqueous solution, and, when calibrated, measures the concentration of the foam forming compounds present in the aqueous solution.
2. Description of the Prior Art
Foam forming compounds include cleaning compounds, such as detergents, fire-fighting chemicals, and naturally occurring surfactants, such as plant extractives. The presence of foam forming compounds can interfere with the operation of chemical plants, such as wastewater treatment plants, by causing inaccurate readings in flow and level sensing devices.
Foaming of wastewater tends to lift solid materials out of the liquid phase and suspend the materials in the foam. These solid materials may include metals or other hazardous materials. In open top tanks, pollutant-laden foams may be blown off the surface of the wastewater and onto the surrounding property. Hazards of this type often result in citations from public health offices and environmental protection officials.
Some foam forming chemical are also toxic to the microorganisms used in wastewater treatment plants. Early detection of foam forming chemicals permits process streams contaminated with these chemicals to be diverted from the main process flow. The diverted flow can be subsequently treated in a specialized foam forming agent removal process.
Foam detecting devices used in the past to detect the presence of foam forming chemicals in an aqueous solution cannot rapidly detect a change of state from a foaming input stream to a non-foaming input stream. For example, if a prior art device was measuring the foam forming characteristic of an input stream that contained a high concentration of a foam forming chemical, and then the input stream was changed to a stream that contained little or no foam forming chemical, the prior art device could not rapidly detect the change in input stream composition. This is because the prior art device has a fixed or static solution reservoir at the bottom of the device, and the concentration of the foam forming chemical in that reservoir is changed only by dilution from the input stream. It may take several minutes before a low concentration input stream dilutes the solution in the reservoir to a concentration that no longer forms a significant amount of foam.
Devices used in the past to detect the presence of foam forming chemicals are generally not automated. These devices are manually operated and are best suited to a laboratory environment.
Prior art devices for detecting the presence of foam forming chemicals are also fragile, generally consisting of a piece of custom blown glasswork.
In addition, prior art devices rely on photo-optical sensor pairs to detect and measure the presence of foam at discrete locations. This approach is expensive to implement and provides a limited number of foam height detection values. Also, reliance upon photo-optical pairs to detect the present of foam requires that the column containing the foam be transparent. In some foam sensing applications, a film of oil, algae, bacteria, and other deposits may eventually occlude a clear column. This renders the photo-optical sensors inoperable.
Further, at low concentrations of foam forming chemical, the foam can usually be characterized as being composed of a small number of large bubbles. The beam from a photo-optical sensor can intermittently pass through such loosely structured foam, resulting in intermittent false readings of foam height.
Accordingly, there is a need for an apparatus for detecting and measuring foam forming compounds in aqueous solutions which is accurate, relatively simple in design, and sufficiently strong to avoid breakage and low cost.
A sample of the liquid or wastewater to be tested enters a vertically positioned tubular column from a fill valve through a column cap at the top of the tubular column, flows down the sides of the column, and collects in a lower portion of the column. The liquid level in the column rises to a liquid level switch. Closing the liquid level switch prevents further flow of liquid into the tubular column.
After a sample of liquid has collected in the lower portion of the column, an air pump is actuated and compressed air flows into the sample through an aeration stone. The air bubbles produced by the aeration stone cause the foam forming compounds in the sample to produce foam. The foam rises in the column and lifts a float which functions as a solid target for an acoustic distance measuring device. The measuring device measures height within the column, generating a continuous analog electrical output signal which is a function of foam height. The value of voltage produced by the measuring device is measured and retained by a programmable logic controller connected to the measuring device.
As the float rises in the sensor tube, a beam of light between photo-optical sensors is encountered and is broken. As the float passes the beam of light, the beam then encounters the foam in the tubular column. If the foam is of sufficient density that it continues to interrupt the beam of light and it continues to lift the ball to a lower set point programmed into the measuring device, a red indicator light is illuminated. If the foam density is insufficient to block the beam of light generated by the optical sensors, the red indicator light does not illuminate and the system recognizes that the aqueous foam forming film concentration is below a predetermined threshold level. When the red indicator light remains illuminated, it indicates that the sample solution contains aqueous foam forming film at or above a predetermined threshold and the apparatus automatically sends a message to alert the user.
After a foam height measurement has been made, the fill valve closes, a sample drain valve opens, and a 3-way valve is positioned to divert compressed air from the aeration stone to the top of the column. This forces the sample of solution and foam from the column through a drain valve. After the solution has been drained from the column, the fill valve opens, the drain valve closes, air is re-directed to the aeration stone, and the entire sample acquisition and measurement cycle is repeated.